Ported from amal66/mike#24 onto current main; lockfiles regenerated against
this tree. Adds vitest as a dev dependency with a `test` script in both
packages, excludes test files from the backend tsc build, and seeds one
suite per package (backend: downloadTokens, 12 tests; frontend: cn() utils,
8 tests). Verified locally: backend 12/12, frontend 8/8 passing.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>